Passive termination methods for a Small Computer Systems Interface (SCSI) have traditionally provided reliable operation at low data transfer rates. At higher data transfer rates, however, transmission line effects become troublesome and passive terminators do not adequately compensate for impedance mismatches. Voltage reflections due to an impedance mismatch between a SCSI terminator and a transmission line are detrimental to overall noise immunity of the system and become increasingly troublesome the higher data transfer rates become. Also, at higher data transfer rates, the rising or falling edge of a signal can have a duration that comprises a significant portion of the signal duty cycle. This imposes tighter system timing constraints.
Another drawback associated with the use of passive terminators involves wasted power. Because a DC path to ground always exists in a passive terminator, the passive terminator continuously dissipates power, even when the signal line is inactive. Additionally, since the passive terminator Thevenin voltage is unregulated, the noise immunity of the system is compromised and the maximum limit of source current specified by the SCSI standard is impinged upon. FIG. 1 shows a passive terminator I-V characteristic 100.
Boulay terminators that utilize an active voltage regulator to maintain a constant voltage at one terminal of a 110.OMEGA. resistor, where the other resistor terminal couples to the line, are known in the art. Because the Thevenin voltage is regulated, the output current is essentially immune to bias supply variations. Consequently, increased noise immunity is achieved as compared to passive terminators. The average power dissipation of the Boulay terminator is less than the power dissipated in a passive terminator, since the only power dissipated in the Boulay terminator is produced by the current required to power the regulator.
Although the Boulay terminator improves noise immunity and reduces power dissipation of a SCSI terminator, it does not address the timing issues associated with the rising or falling edge rates of a signal due to line and other parasitic capacitances. If the rising or falling edge duration becomes comparable in duration to the signal duty cycle and there exists ringing on the transient edge because of impedance mismatches, erroneous sampling may result. FIG. 1 shows a Boulay terminator I-V characteristic 102.
An active element in the termination path between the regulated voltage reference and the SCSI line provides a means for decreasing the transient time after a low-to-high transition as compared to what can be obtained using a passive element. While this may adversely affect transient performance after a high-to-low transition as compared to what can be obtained using a passive element, the trade-off is a good one as the transient performance after a low-to-high transition is more important than transient performance after a high-to-low transition because clock signals are positive edge triggered. Referring to FIG. 1, simulations suggest that Boulay I-V characteristics 102 are desirable for signal line assertions (high-to-low transitions) and "ideal" current source I-V characteristics 104 are desirable for signal line negations (low-to-high transitions). This implies that an optimum terminator I-V characteristic falls somewhere in between these two I-V characteristics. By using a MOSFET as an active element to replace the 110.OMEGA. resistor of the Boulay-type terminator, together with an amplifier configured with feedback architecture, a customizable I-V characteristic 106 can be achieved. Such an active SCSI termination technique is described in U.S. patent application Ser. No. 08/267,119, filed on Jun. 27, 1994.
An active SCSI termination circuit is illustrated in FIG. 2. The active termination circuit provides a means for achieving an optimum terminator I-V characteristic, however, it does not provide means for safely accomplishing a "hot insertion." Therefore an object of the present invention is to provide a hot insertable SCSI terminator having an optimum I-V characteristic.
Hot insertion of a SCSI device refers to the act of coupling a SCSI device, which is initially unpowered, to a powered SCSI bus. It is important to prevent the inserted device from drawing excessive currents during a hot insertion. Excessive currents are currents that may cause device damage or threaten data integrity. This is true not only to protect the inserted device from damage, but also to insure that other devices already coupled to the SCSI bus are not damaged and to ensure that the data integrity on the SCSI bus is not compromised.
Immediately following a hot insertion, the potential at the reference node of the terminating circuit, VREF in FIG. 2, is zero. A finite time period is subsequently required before the circuitry that generates VREF (a voltage regulator powered by a power line from the SCSI bus, TERMPWR) establishes a constant, steady state reference voltage of approximately 2.85 volts at VREF.
During a hot insertion, a SCSI device and/or its associated terminating circuit may be damaged if the prior art terminating scheme shown in FIG. 2 is utilized. For example, if a SCSI device having an active terminator, as shown in FIG. 2, were hot inserted into a SCSI bus, and a signal line (i.e. one of L1 through L9) was in a high state when the insertion took place, a large transient current could flow through the terminating MOSFET associated with that line since the MOSFET source, which is coupled to the reference node, VREF, is initially at ground potential. This current may be large enough to damage the terminating MOSFET or even the SCSI device itself. Thus, hot insertion of a SCSI device without transient power up protection can be deleterious to both the inserted device and the system.
Hot insertion in the absence of transient power up protection may also lead to a compromise of integrity of data on the SCSI bus. For example, if L1 is high and L2 is low immediately following a hot insertion, the potentially large current drawn from the L1 line during the power up transient might flow into the L2 line, disrupting the high state of L1 and/or the low state of L2, and possibly leading to a data error on the SCSI signal lines.
For these reasons, the use of the termination circuit of FIG. 2 requires the user to power down the system before a SCSI device may be safely coupled to a SCSI bus. In other words, because the SCSI terminator of FIG. 2 does not provide any protection against excessive line currents during the power up transient and does not provide means for protecting the data integrity of the SCSI bus during the power up transient, it cannot be reliably used to terminate a device that may be hot inserted into the SCSI bus.
The precautionary system power-down step in the SCSI device insertion procedure described above may be cumbersome and time consuming or may be inadvertently skipped. Therefore, what is needed is an active SCSI termination circuit capable of coupling to a SCSI bus while the SCSI bus is powered and while the SCSI device to be inserted is not powered. In other words, what is needed is an active SCSI termination circuit capable of being hot inserted into a SCSI bus, that provides transient protection to the inserted device and to the SCSI system until the reference voltage, VREF, at the reference node of the SCSI terminator, has powered up to its steady state value.